1891.] 



Quadrant Electrometers. 



53 



IV. "Quadrant Electrometers." By W. E. AYRTON, F.R.S., 

 J. PERRY, F.R.S., and W. E. SUMPNER, D.Sc. Received 

 May 19, 1891. 



(Abstract.) 



In 1886 it was noticed, on continuously charging up the needle 

 of Sir William Thomson's bifilar suspension quadrant electrometer 

 No. 5, made by Messrs. White, of Glasgow, and in use at the labo- 

 ratories at the Central Institution, that the deflection of the needle, 

 when the same P.D. (potential difference) was maintained between the 

 quadrants instead of steadily increasing, first increased, and then 

 diminished ; so that, both for a large charge on the needle as well as 

 for a small, the sensibility of the instrument was small. A similar 

 effect had been described by Dr. J. Hopkinson, in the ' Proceedings 

 of the Physical Society,' vol. 7, Part 1, for the' previous year, and 

 the explanation he gives of this curious result is, that if the aluminium 

 needle be below the centre of the quadrants, the downward attrac- 

 tion of the needle which varies with the square of the needle's 

 charge increases the pull on the bifilar suspension, and so for high 

 charges more than compensates for the increased deflecting couple 

 due to electrical action. On raising, however, the needle of our 

 electrometer much above the centre of the quadrants, the anomalous 

 variation of sensibility of the instrument with increase of charge in 

 the needle did not disappear, and even when the needle was raised 

 so that it was very close to the top of the quadrants, and when, if 

 Dr. Hopkinson's explanation were correct, the sensibility (or deflection 

 corresponding with a given P.D. between the quadrants) ought to 

 have been very great for a large charge on the needle, it was, on the 

 contrary, found to be small. 



The needle was carefully weighed, with the platinum wire attached 

 and the weight dipping into the acid, and a calculation was made as 

 to the magnitude of the effect that should arise from the change of 

 the pull of the fibres due to any upward or downward attraction of 

 the needle by the quadrants. This calculation showed that for a 

 P.D. of 3000 volts between the needle and the quadrants, the amount 

 of such attraction was quite unable to account for the observed dimi- 

 nution of sensibility with large charges in the needle. Dr. Hopkin- 

 son says in his paper, " Increased tension of the fibres from electrical 

 attraction does not therefore account for the whole of the facts, 

 although it does play the principal part." The experiments that we 

 made at the end of 1886 and beginning of 1887, confirmed by the 

 calculation above referred to, proved that, at any rate in our speci- 

 men of the quadrant electrometer, the principal part of the anomalous 



